The present research program is directed toward a broad investigation of neuronal response plasticity generated by increased synaptic use. This study will be conducted within a simple reflex pathway (acoustic reflex) in a simplified preparation (acute decerebrate cat) in order to facilitate analyses of response modulation during repeated presentations of a single stimulus (habituation) or of integrating stimuli (dishabituation, conditioning). The electrophysiological behavior of specific sensory (cochlear nucleus), non-specific sensory (adjacent reticular formation) and motor (facial nucleus) neurons will be studied during these stimulation procedures so as to determine and contrast the mechanisms promoting response modulation in each cell type and to characterize the activity patterns of the most "plastic" neurons. These studies should, at least in part, answer the question of whether increased synaptic use by one, or by two interacting stimuli, produces an increase or decrease in synaptic potency as well as indicate whether some neurons are more "plastic" than other neurons. Moreover, by utilizing the same system in all the stimulation procedures, it should be possible, to relate or to distinguish between the mechanisms promoting habituation, dishabituation and conditioning. Thus, this experimental situation provides a seemingly unique opportunity to analyze and compare the basic processes which underlie mammalian response plasticity at successively complex organizational levels during short-term or long-term stimulation procedures under conditions which can be maintained at a well controlled and stable level. The intact implanted cat adds an additional dimension, in that it provides a means of studying response modulations over ultra-long periods of time. Thus during successive days of repeated acoustic habituation, studies of the same neuron system utilized in the decerebrate cat will be made. These studies should give some indication of amount of "habituation storage" possible for a given learning situation and suggest possible loci and mechanisms of the information retention.